As an approach for increasing the recording capacity of an optical disc, there are known the techniques of increasing the recording density of an optical disc and of increasing the number of recording layers. In the process of increasing the recording capacity e.g. from CD to DVD or from DVD to Blu-ray Disc, a high-density optical disc is realized by modifying the structure of an optical disc by e.g. forming a recording layer of an optical disc at a position closer to a surface of the optical disc, in addition to modifying the arrangement of an optical pickup by e.g. increasing the numerical aperture of an objective lens and using a short wavelength as the laser wavelength.
In recent years, high-density information recording or reproducing has been possible while keeping the arrangement of an optical system in an optical pickup and the configuration of an optical disc unchanged, owing to the development of the reproduction signal processing technology. There has been proposed an optical disc having a recording density higher than the recording density of Blu-ray Disc capable of recording or reproducing information, using an optical system compatible with Blu-ray Disc. If the recording density differs, the correspondence between the address on an optical disc, and the actual radial position on the optical disc changes. In view of the above, in the case where an optical disc having a recording density different from the recording density of a previously loaded optical disc is loaded in the optical disc device, it is necessary to discriminate the recording densities of the optical discs from each other.
As a conventional example of optical discs having recording densities different from each other and capable of recording or reproducing information by a common optical system, there are known a single-layer DVD and a dual-layer DVD. The recording capacity of the single-layer DVD is 4.7 GBytes, and the recording capacity of the dual-layer DVD is 8.5 GBytes. The recording density of the dual-layer DVD per layer differs from that of the single-layer DVD by about 10%.
In the above example, the recording layer number and the recording density have one-to-one correspondence, and it is possible to easily discriminate the recording densities from each other by counting the number of recording layers. Thus, it is possible to discriminate the recording densities from each other before information is recorded or reproduced.
In the case of Blu-ray Disc, a physical information area recorded with physical information unique to the optical disc is formed in an inner periphery of the optical disc. The physical information area is formed by a track pitch and an address format different from those for a user data area. It is possible to obtain the recording layer number and the recording density of the optical disc loaded in the optical disc device by reproducing the physical information recorded in the physical information area.
Further, patent literature 1 proposes a method for discriminating the recording density of an optical disc before the physical information is reproduced by moving an optical pickup to a specific radial position and by rotating a spindle motor at a specific rotation speed for measuring the frequency of a wobbling groove formed in the optical disc.
On the other hand, there may be a case that it is impossible to accurately count the recording layer number in advance, or it is impossible to accurately discriminate the recording density.
As a method for counting the recording layer number, the counting can be realized by counting the number of focus error signals (hereinafter, called as FE signals) obtained when an objective lens is moved toward a recording layer of an optical disc in a state that laser light is emitted.
However, there is a case that it is difficult to accurately count the recording layer number, e.g. in the case where the recording layer number is increased to three layers or four layers.
In an optical system configured such that the numerical aperture of an objective lens is increased and a short wavelength is used as the laser wavelength so that the optical system is compatible with Blu-ray Disc, it is necessary to properly correct the spherical aberration which changes depending on the thickness of a cover layer of an optical disc. If the spherical aberration of a specific recording layer of plural recording layers is corrected to optimize the spherical aberration, the other recording layers may have an intolerably large spherical aberration, which may result in serious deterioration of the amplitude of an FE signal obtained from the other recording layers.
FIG. 6 is a diagram showing an example of an arrangement on a surface and recording layers of each of a single-layer optical disc, a dual-layer optical disc, a triple-layer optical disc and a quadruple-layer optical disc with respect to the thickness direction thereof. In the dual-layer optical disc, the triple-layer optical disc and the quadruple-layer optical disc, the second to fourth recording layers are formed in this order with respect to the position where the first recording layer as a recording layer farthest from the surface is formed.
The single-layer optical disc has a first recording layer. The dual-layer optical disc has a first recording layer farthest from a surface (light incident surface), and a second recording layer closest to the surface. The triple-layer optical disc has a first recording layer farthest from a surface, a second recording layer second farthest from the surface and a third recording layer closest to the surface. The quadruple-layer optical disc has a first recording layer farthest from a surface, a second recording layer second farthest from the surface, a third recording layer third farthest from the surface and a fourth recording layer closest to the surface.
Taking into account the compatibility with an existing optical disc, the layer structure of a multi-layer optical disc is such that the second to fourth recording layers of each of the dual-layer optical disc, the triple-layer optical disc and the quadruple-layer optical disc are disposed relative to the first recording layer corresponding to the first recording layer of the single-layer optical disc. Specifically, the distance from the surface of each of the dual-layer optical disc, the triple-layer optical disc and the quadruple-layer optical disc to the first recording layer of the each respective optical disc is the same as the distance from the surface of the single-layer optical disc to the first recording layer thereof.
FIG. 7 and FIG. 8 are diagrams showing FE signals detected in the case where the spherical aberration of each respective optical disc is corrected to optimize the spherical aberration of the recording layer farthest from the surface of the each respective optical disc, and the objective lens is moved toward the each respective optical disc in a state that laser light is emitted. FIG. 7 is a diagram showing FE signals detected from a triple-layer optical disc, and FIG. 8 is a diagram showing FE signals detected from a quadruple-layer optical disc.
Generally, if the reflectance difference between recording layers is reduced, while increasing the recording layer number, the reflectance is lowered as the recording layer number is increased. Taking into account the aforementioned influence by spherical aberration, there exists a recording layer whose FE signal amplitude is smaller than that on the surface of the optical disc.
Referring to FIG. 7, FE signals 201, 202, 203, 204 are respectively FE signals obtained from the surface, the first recording layer, the second recording layer and the third recording layer. The presence or absence of a recording layer is determined by judging whether the level of the FE signal has exceeded a plus detection threshold value 205 and a minus detection threshold value 206. In this case, there are four FE signals which have exceeded the detection threshold values, including an FE signal corresponding to the surface, and it is determined that the optical disc has three recording layers.
In FIG. 8, FE signals 301, 302, 303, 304, 305 are respectively FE signals obtained from the surface, the first recording layer, the second recording layer, the third recording layer and the fourth recording layer. The presence or absence of a recording layer is determined by judging whether the level of the FE signal has exceeded the plus detection threshold value 205 and the minus detection threshold value 206. In this case, the FE signal 305 obtained from the fourth recording layer does not exceed the plus detection threshold value 205 and the minus detection threshold value 206. Accordingly, there are four FE signals that have exceeded the detection threshold values, including an FE signal corresponding to the surface, and it is erroneously determined that the optical disc has three recording layers.
As described above, as the recording layer number is increased, the layer interval between the recording layer closest to the surface of the optical disc and the recording layer farthest from the surface of the optical disc is increased. As a result, deterioration in the FE signal amplitude resulting from spherical aberration is increased, and it is impossible to precisely discriminate the recording layer number by FE signals.
It may be possible to enhance the detection precision on the recording layer number by repeating an operation of measuring an FE signal by driving the objective lens while correcting an estimated spherical aberration for each respective recording layer. In this case, however, the larger the recording layer number is, the longer the time required for the measuring operation is.
Further, it is possible to discriminate the recording densities from each other by the aforementioned method of measuring the frequency of a wobbling groove, in the case where the recording density difference is sufficiently large. However, taking into account the precision in moving the optical pickup in the radial direction of an optical disc, an error in the rotation speed of the spindle motor, and an influence by rotational variation of the spindle motor, it is difficult to precisely discriminate the recording densities from each other, in the case where the recording density difference is small.
Next, there is described a problem to be solved in an optical disc compatible with an optical system for Blu-ray Disc in increasing the recording density of the optical disc.
Even in the case where the recording densities differ from each other, it is desirable to dispose various areas on an optical disc in accordance with the recording density of an existing optical disc. In particular, it is desirable to dispose areas having different physical properties from each other at a substantially same radial position.
Specifically, the physical information area on Blu-ray Disc is formed by a track pitch and an address format different from those for the user data area. Accordingly, it is impossible to read the address of the physical information area even if a seek operation is performed in the same manner as for the user data area. As a result, in the case where the physical information area on an optical disc having a recording density different from the recording density of an existing optical disc is formed at a different radial position, and laser light is inadvertently irradiated onto the physical information area of the optical disc, the address of the physical information area may not be read, which makes it impossible to perform an operation thereafter.
If the recording densities of two optical discs differ from each other, the addresses at each respective radial position on the two optical discs inevitably differ from each other. This means that the addresses of the physical information areas on optical discs having recording densities different from each other differ from each other with respect to the innermost periphery of the user data area, despite that the physical information of these two optical discs exists at the same radial position.
The address information representing the address of information to be read out is necessary to read out the information from the optical disc. However, it is impossible to accurately obtain the address of the physical information area in a condition that the recording density cannot be accurately discriminated.
For instance, let it be assumed that the address of a physical information area is set and a seek operation is performed, based on the premise that the recording density of an optical disc loaded in an optical disc device has a recording density different from the actual recording density of the optical disc. In this case, since there is no physical information in the area sought by the seek operation, it is impossible to obtain the physical information unique to the loaded optical disc, which makes it impossible to perform an operation thereafter.